1. Field of the Invention
The present invention relates to repeaters in a digitally-operated radio relay station and more particularly to repeaters that must pass a digital service channel.
2. Description of the Prior Art
In repeaters of digitally-operated radio relay sections, the signal to be transmitted is transferred in the base band or in the intermediate frequency level. In this case, the transmitting receiver and the receiving transmitter operate either synchronously or plesiochronously with respect to each other.
Whereas, in the synchronous transmission process, the phase relationship between the transmit and receive clocks is random, in a plesiochronous operation of the transmitter and receiver, there is a deviation of the clock frequencies from each other, which can reach an order of magnitude of approximately 10.sup.-5. Both phenomena lead to disturbances in the switching of digital service channels, which are intended to be accessible in the repeater, and therefore are removed from the signal to be transmitted and later reinserted into it.
These disturbances originate from the fact that, at points at which an edge (control edge) of a receive clock obtained from the received signal that is controlling the sampling of an incoming digital service-channel signal coincides in time with an edge (control edge) of a transmit clock controlling the re-insertion of the digital service-channel signal, the assignment of the digital service-channel signal bits that are to be inserted into the signal being transmitted to the transmit clock is not clear-cut.
This condition occurs with a certain probability after each new start-up during the synchronous operation of the transmitter and the receiver and random phase of the clocks with respect to each other. During plesiochronous operation, it occurs in all cases, with the pulse repetition rate of the differential frequency of the two clocks.
Avoidance of these disturbances, up to now, has required special adaptation measures, such as the interposition of elastic storages for the insertion of padding bits into the pulse frame to be transmitted, for extraction of data bits from it (see, for example, publication ANT 166903.87 of ANT Nachrichtentechnik GmBH, page 2, middle column).
Measures of this type are time-consuming and therefore expensive and liable to malfunction.